1. Field of the Invention
The present invention relates to a method for polymerizing olefins such as ethylene and propylene, and more particularly to a method for polymerizing olefins in a multi-stage polymerization apparatus comprising a gas-phase polymerization reactor in which the composition of gas is easily controlled.
2. Background Art
Among a variety of industrial methods for polymerizing olefins such as ethylene and propylene, a gas-phase polymerization method has become of interest. This method utilizes high-activity catalyst and does not require a deashing (removal of catalyst residue) process.
However, in the case of a successive multi-stage gas-phase polymerization method, the composition of gas in a polymerization reactor of a certain stage affects the composition of gas in a polymerization reactor of a subsequent stage. Therefore, adjustment of the composition of gas in the reactor of a subsequent stage may be difficult.
For example, in order to produce a polymer having a broad molecular weight distribution, usually a low molecular weight polymer is produced in a reactor of a certain stage, and in a reactor of a subsequent stage the resultant polymer is further polymerized to yield a higher polymer. In this case, in the preceding polymerization, hydrogen gas of high concentration is used as a chain-transfer agent. However, since hydrogen gas of low concentration is used in the subsequent polymerization, when the polymer is transferred from a preceding reactor to a subsequent reactor there must be removed a large amount of hydrogen gas accompanying a produced polymer.
In addition, when a copolymer such as ethylene-propylene is to be produced, the composition of gas in each polymerization reactor must be adjusted.
Therefore, several methods have been proposed for adjusting the composition of gas in a gas-phase polymerization reactor of a subsequent stage. Japanese Patent Application Laid-Open (kokai) No. 23001/1984 discloses a method in which a purge vessel is provided between two sequential polymerization reactors for adjusting the concentration of hydrogen gas (a chain-transfer agent) contained in an accompanying gas including a polymer fed from a preceding polymerization reactor, wherein the accompanying gas is removed and only the polymer is transferred to a subsequent reactor. Also, Japanese Patent Application Laid-Open (kokai) No. 65703/1982 discloses a method in which an accompanying gas containing a polymer is diluted with inert gas in a purge vessel, to thereby transfer to a subsequent reactor the polymer accompanied by hydrogen gas of low concentration. Furthermore, Japanese Patent Application Laid-Open (kokai) No. 118342/1995 discloses a method in which a purge vessel is formed of a tank for gas and a tank for powder, and an accompanying gas containing a polymer is removed by operation of a valve between the two tanks, and then the polymer in the tank for powder is transferred to a subsequent polymerization reactor by use of pressurized circulating gas from the subsequent reactor. However, these methods still have drawbacks, such as requirement of a purge vessel and complicated valve control operation.